<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var a = 10;
    function foo(){
        var b = 20;
        console.log('我是函数')
    }
    console.log(window.a);
    console.log(window.foo);
    console.log(window.b);  //b 不是第一层级定义的变量，所以无法读取
</script>
</body>
</html>